Absorbent articles such as diapers, incontinence guards, sanitary napkins, intended to be disposed after one single use, contain an absorbent structure having the capability to acquire large amounts of liquid under a short period of time, having further the ability to distribute the liquid and to store the liquid. This means that the absorbent structure usually comprises several different layers having different properties with respect to each other. It is further desired that such absorbent articles are thin and discrete to wear.
Frequently, the absorbent structure comprises at least a liquid acquisition layer and a liquid storage layer. The liquid storage layer often comprises a composite web structure of fibrous material, usually cellulosic fluff pulp, and particulate superabsorbent material, which is a polymer having the ability to absorb water or bodily fluids many times its own weight. The superabsorbent material is either mixed with the fibrous material or applied in a layered configuration between fibrous layers. The liquid acquisition layer often comprises a porous fibrous layer of wet resilient fibers, such as synthetic fibers, or polymeric foam materials.
Cellulosic fluff pulp of different types are available, such as mechanical, thermomechanical, chemithermomechanical or chemical pulp. Fluff pulp is delivered in bales or rolls and is defibrated in pulp mills before being formed. Mat forming for forming absorbent structures is carried out by transporting the defibrated pulp from the pulp mill in an air flow to a forming unit, usually in the form of what is known as a mat-forming wheel. This is air-permeable and the fibers remain on the periphery of the mat-forming wheel and forms a fiber mat of a low density, which is subsequently compressed.
During mat forming, a continuous web can be formed, which is subsequently compressed and cut to individual absorbent structures adapted to be applied in an absorbent article. Alternatively the mat-forming wheel is provided with a plurality of moulds arranged over the periphery of the wheel so that a plurality of individual absorbent structures of a desired shape and size are formed directly on the mat-forming wheel.
The prior art teaches different types of mat-forming wheels for making absorbent structures. U.S. Pat. No. 3,518,726 discloses mat-forming in moulds, wherein the air flow and the forming in the moulds is controlled by the bottom of the moulds having different hole density and/or different hole size in different areas.
U.S. Pat. No. 4,765,780 discloses that fibers are fed in individual fiber streams to different forming hoods along the periphery of a large mat-forming wheel. Different layers are formed on top of each other as the forming wheel passes the different forming hoods. Superabsorbent particulate material may be added to one of these fiber streams.
In EP-A-1 253 231 there is disclosed a method of forming a fibrous web wherein separate air flows containing fibers are supplied to a number of different mat-forming wheels. Separate web layers are formed on the respective mat-forming wheel and are combined downstreams of the mat-forming wheels to form a common fibrous web, which is subsequently compressed. Superabsorbent particles may be added together with the fibrous streams or separately. The process is told to result in an increased manufacturing quality of the fibrous web formed and an increased manufacturing speed.
The prior art further teaches different methods and devices for applying particulate superabsorbent material in a fibrous web either in a mixed or layered configuration.
U.S. Pat. No. 5,885,516 discloses a method and an apparatus for introducing a superabsorbent powder at an intermediate stage between an initial supply of fibers and the final formation of the composite web. The superabsorbent powder is applied to the fibrous web by spreading the powder by means of a rotating brush cylinder, a perforated compressed air pipe or a pair of rod electrodes.
U.S. Pat. No. 5,429,788 discloses an apparatus and a method for forming a discrete layer of superabsorbent particulate material within a composite web. The particulate material is conveyed by gravity or in a conveying gas stream into a forming chamber. A depositing mechanism is located within the forming chamber to selectively dispense the particulate material in a fibrous web supported by a foraminous conveyor.
U.S. Pat. No. 5,213,817 discloses an apparatus for intermittently applying particulate superabsorbent material on a moving fibrous substrate. The continuous powder stream is transmitted through a nozzle and is split into two streams, one of which is applied to the fibrous substrate and the other is recirculated back to the feeding device.
WO 92/19198 discloses a device for applying a discrete layer of superabsorbent particles on a moving fibrous web, wherein a particle dispenser dispenses superabsorbent particles continuously over a movable belt provided with a pattern of holes, said belt is arranged at a short distance above and moves over the fibrous web, so that the superabsorbent particles are applied to the fibrous web in a pattern corresponding to the pattern of holes in the belt.
Development is now moving towards very thin absorbent structures, which means that a high content of superabsorbent material is required, in order to provide the necessary absorption capacity. It also requires a high manufacturing accuracy in order for the absorbent products to function in the intended manner. The demands of the consumers for a uniform quality and a better functioning in terms of leakage security, fit and comfort are also increasing. Moreover, the requirements for an improved performance and an increased price pressure due to a hard competition result in that the speed of the manufacturing process has to be increased.
An increased manufacturing speed results in that more exact conditions are required in connection with the forming of the absorbent structures contained in the absorbent articles. In combination with the demands for thinner products, this results in that known manufacturing methods are not functioning satisfactorily in all aspects.